U.S. patent application number 10/886769 was filed with the patent office on 2005-01-20 for substrate for magnetic recording medium, method for manufacturing the same and magnetic recording medium.
Invention is credited to Ishii, Masatoshi, Ohashi, Ken, Tsumori, Toshihiro.
Application Number | 20050011860 10/886769 |
Document ID | / |
Family ID | 34055840 |
Filed Date | 2005-01-20 |
United States Patent
Application |
20050011860 |
Kind Code |
A1 |
Ishii, Masatoshi ; et
al. |
January 20, 2005 |
Substrate for magnetic recording medium, method for manufacturing
the same and magnetic recording medium
Abstract
Provided is a substrate for a magnetic recording medium,
preferably a substrate having a small diameter of not more than 65
mm, which is advantageous in respect of physical properties and
cost. More specifically, provided is a substrate for a magnetic
recording medium, using a monocrystalline silicon wafer which has
been heated and/or etched at least once before. Moreover, provided
is a method for manufacturing a substrate for a magnetic recording
medium, the method comprising a step of coring for obtaining a
plurality of doughnut-shaped substrates having an outer diameter of
not more than 65 mm from a monocrystalline silicon wafer having a
diameter of at least 150 mm and at most 300 mm which has undergone
heating and/or etching at least once. The method may preferably
further comprise a step of chamfering for removing edges of inner
and outer circumferential faces of said doughnut-shaped substrate;
and a step of circumferential face-polishing for polishing the
chamfered inner and outer circumferential faces.
Inventors: |
Ishii, Masatoshi;
(Takefu-shi, JP) ; Tsumori, Toshihiro;
(Takefu-shi, JP) ; Ohashi, Ken; (Takefu-shi,
JP) |
Correspondence
Address: |
MYERS BIGEL SIBLEY & SAJOVEC
PO BOX 37428
RALEIGH
NC
27627
US
|
Family ID: |
34055840 |
Appl. No.: |
10/886769 |
Filed: |
July 8, 2004 |
Current U.S.
Class: |
216/56 ; 216/88;
216/99; 428/900 |
Current CPC
Class: |
C30B 33/00 20130101;
C30B 29/06 20130101; C30B 29/60 20130101 |
Class at
Publication: |
216/056 ;
428/900; 216/088; 216/099 |
International
Class: |
B44C 001/22; C03C
025/68; H01L 021/76 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 15, 2003 |
JP |
2003-197118 |
Claims
1. A substrate for a magnetic recording medium, comprising a
monocrystalline silicon wafer which has been heated and/or etched
at least once before.
2. A method for manufacturing a substrate for a magnetic recording
medium, the method comprising: a step of coring for obtaining a
plurality of doughnut-shaped substrates having an outer diameter of
not more than 65 mm from a monocrystalline silicon wafer having a
diameter of at least 150 mm and at most 300 mm which has undergone
heating and/or etching at least once.
3. The method for manufacturing a substrate for a magnetic
recording medium according to claim 2, further comprising: a step
of chamfering for removing edges of inner and outer circumferential
faces of said doughnut-shaped substrate; a step of circumferential
face-polishing for polishing the chamfered inner and outer
circumferential faces; and a step of lapping for removing 10 .mu.m
to 100 .mu.m by grinding from a surface of the monocrystalline
silicon wafer or the doughnut-shaped substrate, before the step of
coring, or between the steps of coring and chamfering, or between
the steps of chamfering and circumferential face-polishing, or
after the step of circumferential face-polishing.
4. The method for manufacturing a substrate for a magnetic
recording medium according to claim 2 wherein said monocrystalline
silicon wafer used in said step of coring has a surface orientation
of (1 0 0) and a thickness of not more than 0.7 mm.
5. The method for manufacturing a substrate for a magnetic
recording medium according to claim 3, further comprising after
said step of circumferential face-polishing, or after said step of
lapping performed after said step of circumferential
face-polishing: a step of alkali-etching the substrate; a step of
polishing both surfaces of the alkali-etched substrate; and a
subsequent step of washing.
6. A perpendicular magnetic recording medium comprising said
substrate of claim 1.
7. The method for manufacturing a substrate for a magnetic
recording medium according to claim 3 wherein said monocrystalline
silicon wafer used in said step of coring has a surface orientation
of (1 0 0) and a thickness of not more than 0.7 mm.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to a recording medium substrate for
magnetic recording, and more specifically to a recording medium
substrate for magnetic recording which is optimal as a small
diameter substrate preferably having a diameter not more than 65 mm
and more preferably having a diameter not more than 50 mm.
[0003] 2. Description of the Related Art
[0004] The increase in recording density (surface density) of
magnetic recording has been extremely rapid, the rapid increase
over these past 10 years advancing continuously at yearly rates of
50 to 200%. At the mass production level, products with 70
Gbits/inch.sup.2 are shipped, while surface recording densities
twice higher, namely 160 Gbits/inch.sup.2, have been reported at
the laboratory level. Surface recording densities at the mass
production level correspond to 80 Gbytes per one platter of a 3.5"
HDD (3.5 inch), and corresponds to 40 Gbytes per single platter of
a 2.5" HDD. At these recording volumes, installation of single
platter recording media gives a sufficient volume for use in an
ordinary desk top personal computer (equipped with a 3.5" HDD) or a
laptop personal computer (equipped with a 2.5" HDD).
[0005] It is expected that recording densities will also continue
to improve in the future. However, conventional horizontal magnetic
recording methods are approaching their thermal fluctuation
recording limit. Thus, when recording densities of 100
Gbit/inch.sup.2 to 200 Gbit/inch.sup.2 are reached, it is believed
that it will be replaced by perpendicular magnetic recording. At
the present time it is not certain what the recording limit of
perpendicular magnetic recording will be, but it is believed that
1000 Gbit/inch.sup.2 (1 Tbit/inch.sup.2) is achievable. If these
types of high recording densities are achieved, it will be possible
to obtain a recording volume of 600 to 700 Gbytes per single
platter of a 2.5" HDD.
[0006] As it is very likely that such a large volume will not be
fully utilized by ordinary personal computer use, recording media
having a diameter smaller than 2.5" are gradually coming into use.
Typically, there are substrates of 1.8" or 1", and 1.3" HDDs was
also sold in the past. HDDs of not more than 2" have very small
capacities at the present time, however if magnetic recording
densities increase in the future, then a 1.8" HDD in a personal
computer (particularly in a laptop) can ensure a sufficient
recording volume. Furthermore, the recording volume of a 1" HDD is
in the order of 1 to 4 Gbyte at the present, however if the volume
was several times larger, many possibilities for a wide range of
mobile uses would emerge, not limited just to digital cameras and
the like, but also for personal computers and digital video
cameras, information terminals, hand held music devices and mobile
phones for example. Small diameter HDDs, small diameter recording
media and substrates having diameter of not more than 2" offer
promising applications in the future.
[0007] As a substrate for the recording medium of a HDD, Al alloy
substrates are mainly used for 3.5" substrates, while glass
substrates are mainly used for 2.5" HDDs. There is a high
possibility of HDDs in mobile applications, such as in laptop
computers, receiving a shock. Because the possibility of data loss
from scratches to the recording medium or the head resulting from
head collision is large, the 2.5" HDDs mounted in these devices
have come to use very hard glass substrates. Consequently, there is
also a large possibility that glass substrates will also be used in
small diameter substrates of not more than 2".
[0008] However, because small diameter substrates of not greater
than 2" are mainly used in mobile applications, shock resistance is
of greater importance than for 2.5" substrates mounted in laptop
computers. Furthermore, from the need for the smaller size, there
is a demand for making all parts including the substrate smaller
and thinner. A thickness of the 2" substrate board is demanded that
is even thinner than the 0.635 mm standard thickness of the 2.5"
substrate. Due to the specifications required of such small
diameter substrates, the demand is for substrates which are easily
fabricated, which have a high Young's modulus and which have
sufficient strength even though thin. Glass substrates have a
number of problems on these points.
[0009] First, when the board thickness of the crystalline glass
substrate which is actually used is not more than 0.635 mm, the
Young's modulus is insufficient and resonance frequencies exist in
the practical rotating region during rotation. Consequently, it is
difficult to slim down further than this. Furthermore, although
glass base plates are already used as substrates with a thickness
in the 0.8 mm range, it is difficult to fabricate glass
compositions which are any thinner than this, as demanded as HDD
base plates. Because of this, it is necessary to adjust the
thickness by lap-grinding from the 0.8 mm range down to the 0.5 mm
range or even thinner. This is not preferable as it increases
process costs and process time because the polishing time for width
adjustment becomes very long.
[0010] Furthermore, the glass substrate is naturally a
non-conductor, so there is the problem of charge up on the
substrate when making films by sputtering. Thus, it is necessary to
insert a metal film buffer between the substrate and the magnetic
film in order to ensure favorable contact with the magnetic film.
Basically, these technical problems have been solved, however this
is one reason why it is difficult to use glass substrates in a
sputter film forming process. Because of this, it would be ideal if
it were possible to confer conductivity to the substrate, however
this is difficult with glass substrates.
[0011] Just as glass substrates are mainly used even in 2.5" HDDs,
Al alloy substrates are completely unsuitable for mobile
applications. It was stated previously that the hardness of the
substrate is insufficient. Because substrate stiffness is also
insufficient, the only way to ensure that resonance frequencies are
above the actual rotating region is to increase the thickness.
Thus, it is not possible to consider it as a candidate substrate
for mobile applications.
[0012] A number of other substitute substrates have been proposed,
such as sapphire glass, SiC substrates, engineering plastic
substrates, carbon substrates and the like, however from the
standard evaluations of strength, processability, cost, surface
smoothness and compatibility for film deposition and the like, all
are inadequate as substitute substrates for small diameter
substrates.
[0013] Use of a Si monocrystalline substrate has been proposed as a
HDD recording film substrate (Japanese Patent Provisional
Publication No.6-176339/1994). A Si monocrystalline substrate is
superior as the HDD substrate because of its excellent substrate
smoothness, environmental stability and reliability, and because
its stiffness is also comparatively high when compared to a glass
substrates. Differing from a glass substrate, it has at least the
conductivity of a semi-conductor. Furthermore, because it is
generally the case that a regular wafer includes P-type or N-type
dopant, the conductivity is even higher. Consequently, there is no
problem with charge-up during sputter film formation as with glass
substrates, and it is possible to sputter a metal film directly
onto the Si substrate. Furthermore, because it has favorable
thermal conductivity, the substrate is easily heated, film
formation is possible even at high temperatures above 300.degree.
C. and it is excellently suited to the sputter film forming
process. Si monocrystalline substrates for semi-conductor IC use
are mass-produced as wafers having a diameter of 100 mm to 300
mm.
SUMMARY OF THE INVENTION
[0014] However, it is currently difficult to obtain a small
diameter wafer having a diameter of not more than 100 mm.
Consequently, it is practical to cut out the desired small diameter
substrate by coring from a 6" or 8" wafer, which is currently in
most common use. However, because the price of semi-conductor grade
Si monocrystalline substrate is expensive, it is at a noticeable
disadvantage compared to the glass base plates or the Al base
plates from a cost aspect.
[0015] The invention provides a substrate for a magnetic recording
medium, preferably a small diameter substrate having a diameter of
preferably not more than 65 mm, more preferably not more than 50
mm, which is advantageous with regard to physical properties and
cost.
[0016] The invention provides a substrate for a magnetic recording
medium using a monocrystalline silicon wafer which has been heated
and/or etched at least once.
[0017] The invention also provides a method for manufacturing a
substrate for a magnetic recording medium, the method comprising a
step of coring, wherein a plurality of doughnut-shaped substrates
having an outer diameter of not more than 65 mm and a preferable
inner diameter of not more than 20 mm, a more preferable inner
diameter of not more than 12 mm, are obtained by coring of a
monocrystalline silicon wafer having a diameter of at least 150 mm
and at most 300 mm which has undergone heating and/or etching at
least once. The method may preferably further comprise a step of
chamfering wherein edges of inner and outer circumferential faces
of said doughnut-shaped substrate are removed; and a step of
circumferential face-polishing wherein the chamfered inner and
outer circumferential faces are polished (or ground). The method
may preferably comprise a step of lapping wherein 10 .mu.m to 100
.mu.m is removed by polishing (or grinding) from a surface of the
monocrystalline silicon wafer or the doughnut-shaped substrate,
preferably before or after the step of coring. The step of lapping
may be comprised, for example, preferably before the step of
coring, between the steps of coring and chamfering, between the
steps of chamfering and circumferential face-polishing, or after
the step of circumferential face-polishing. The step of lapping may
be comprised more preferably before the step of coring, between the
steps of chamfering and circumferential face-polishing, or after
the step of circumferential face-polishing.
[0018] According to the invention, a silicon monocrystalline
substrate is provided that is appropriate for a substrate for a HDD
magnetic recording medium. The substrate is advantageous from the
aspect of physical properties and cost.
BRIEF DESCRIPTION OF THE DRAWING
[0019] FIG. 1 shows a scheme of one example for producing a
substrate for a HDD magnetic recording medium, using a silicon
monocrystalline wafer as a base plate.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] The invention relates to a substrate for a HDD magnetic
recording medium comprising a Si monocrystalline substrate having a
diameter of not more than 65 mm (the diameter described here is the
nominal diameter) which is fabricated by a coring process from a
silicon monocrystalline wafer which has undergone thermal treatment
and/or etching at least once; and a method for manufacturing the
same.
[0021] FIG. 1 shows a scheme of one example for producing a
substrate for a HDD magnetic recording medium, using a silicon
monocrystalline wafer as a base plate.
[0022] A monocrystalline silicon rod 1 is sliced to produce
monocrystalline silicon wafers 2 having a diameter of 200 mm and
cored to obtain doughnut-shaped wafers 3 having an outer diameter
of 65 mm. According to a method disclosed in Japanese Patent
Provisional Publication No. 10-334461/1998, seven cores of HDD
substrates having a diameter of 65 mm can be obtained from a 200 mm
monocrystalline silicon wafer. The edges of inner and outer
circumferential faces of the doughnut-shaped substrate 3 may be
preferably removed and the circumferential faces are polished.
Subsequently, the small diameter substrate may be produced
typically by a step of alkali etching, a step of polishing both
surfaces and a step of washing.
[0023] A step of lapping for grinding off preferably 10 .mu.m to
100 .mu.m of the surface of the monocrystalline silicon wafer or
the doughnut-shaped substrate may be comprised before or after the
step of coring, for example before the step of coring, between the
step of coring and the step of chamfering, between the step of
chamfering and the step of circumferential face-polishing, or after
the step of circumferential face-polishing. The step of lapping may
be more preferably comprised before the step of coring, between the
step of chamfering and the step of circumferential face-polishing,
or after the step of circumferential face-polishing.
[0024] The monocrystalline silicon wafer used in the step of coring
may preferably have a surface orientation of (1 0 0), an outer
diameter of at least 150 mm and at most 300 mm and have a thickness
of 0.4 mm to 1 mm (more preferably 0.7 mm or less).
[0025] Semiconductor grade silicon monocrystalline wafers (prime
wafers) are very expensive, so if a 65 mm diameter substrate is
fabricated using a monocrystalline base plate, it will cost from a
few times closely to ten times the cost of a glass substrate. No
matter how much better the characteristic properties of the silicon
monocrystalline substrate are, just this cost difference alone
makes it difficult to put these to practical use.
[0026] On the other hand, a silicon monocrystalline monitor wafer
of the same diameter is used for the purpose of monitoring the
steps in the semiconductor IC process. The ratio of monitor wafers
to prime wafers may approach 1:1 when starting a new diameter
substrate or a process thereof. Although the quality of monitor
wafers is not inferior to that of the prime wafers, they are
slightly inexpensive. Use of monitor wafers in the manufacturing
process of semiconductor ICs is unavoidable, it is difficult to use
the desired number of the substrates in substrates for HDDs, and
there is no big cost reduction even at the price of the base
plates.
[0027] In addition to prime wafers and monitor wafers, dummy wafers
(or recycled wafers) are used as the Si monocrystalline substrate
in the semiconductor IC process. Dummy wafers are re-used at least
once for the purpose of checking or investigating the process.
Monitor wafers which have been used once and which have had their
oxide layer or metal layer scraped off are recycled wafers.
Although obvious, the price of substrates is in the sequence of
prime wafers>monitor wafers>recycled wafers. Recycled wafers
are ordinarily used at least once, up to the order of 5 to 6 times,
and the variety of adherent films on the upper surface is polished
off each time they are used. Because the recycled wafer is
gradually ground away, it becomes thinner by an order of 10 .mu.m
to 100 .mu.m with each use. Because recycled wafers which have
become thinner than a standard thickness value become unsuitable
for the purpose of checking the process, they are disposed of
without further use. There are a variety of standard thicknesses,
but generally they may be discarded once their thickness reaches
0.7 mm to 0.5 mm or less.
[0028] Because recycled wafers are used many times over, they
experience a variety of steps in the semiconductor process.
Consequently, because each wafer also acquires a variety of thermal
histories, various types of ion implantations, dopants and
electrical resistances and the like, it become unsuitable for use
in solar battery applications and the like so that it is usually
discarded.
[0029] In silicon monocrystalline substrates for HDDs, the thermal
history of the monocrystalline wafer base plate, or the type of
dopants used or the like are not important. Irrespective of N-types
or P-types, as long as it has at least the conductivity of a
semiconductor it is applicable. It is important that it is a
monocrystal which has no grain boundary on the polished face. As a
HDD substrate, the important points are surface smoothness after
polishing, and the strength required for the substrate. There is no
change to the fact that recycled wafers are still silicon
monocrystals, so there is absolutely no problem with surface
smoothness after polishing.
[0030] Because recycled wafers have been heated at least once, the
substrate strength may be reduced due to crystal defects or
dislocations at the atomic level, or by micro-scratches or
micro-cracks when the HDD substrate is processed. However, it has
been found that as long as these defects do not exist at the
substrate surface, on the contrary the strength of the substrate
becomes stronger. The reason for this is not clear, however it
appears that dissolved oxygen links with one portion of the silicon
and behaves as a supporting member. Thus, it is possible to use
discarded recycled wafers having less than a predetermined
thickness as a relatively low cost HDD base substrate.
[0031] The invention provides a method for manufacturing a
substrate for a magnetic recording medium comprising a step of
coring wherein a plurality of doughnut-shaped substrates having an
outer diameter of not more than 65 mm are obtained by coring of a
monocrystalline silicon wafer having a diameter of at least 150 mm
and at most 300 mm which has undergone heating and/or etching at
least once before, and preferably, a step of chamfering of the
inner and outer circumferential faces of the doughnut-shaped
substrate and a step of circumferential face-polishing.
[0032] The monocrystalline silicon wafer having a diameter of at
least 150 mm and at most 300 mm which has undergone heating and/or
etching at least once before includes recycled wafers and used
monitor wafers, excluding prime wafers.
[0033] The monocrystalline silicon wafer which has undergone
heating at least once may include, for example, wafers that have
been heat treated (for example at 400 to 1350.degree. C.) once as
monitor wafers, and wafers that have been heat treated at least
twice as recycled wafers.
[0034] A monocrystalline silicon wafer which has undergone etching
at least once, may include for example wafers that have undergone
various types of etching once in the semiconductor manufacturing
process as monitor wafers, and wafers that have been etched at
least twice as a recycled wafer.
[0035] In the step of coring, a plurality of substrates having an
outer diameter of not more than 60 mm can be obtained from a
monocrystalline silicon wafer having a diameter of at least 150 mm
and at most 300 mm, using for example cup grinder processing, laser
processing with a CO.sub.2 laser or a YAG laser or the like, water
jet processing using high pressure water mixed with abrasive
material, or blast processing.
[0036] The step of coring may comprise outer diameter coring (outer
circumferential coring) and inner diameter coring (inner
circumferential coring).
[0037] In the case of cup grinder processing, it may be more
efficient to carry out the outer diameter coring following the
inner diameter coring, wherein the inner diameter core portion is
used as a hold-down hole during the outer coring. It is because,
for example, only the material that has passed a predetermined
inspection after the inner diameter coring can be subjected to the
outer diameter coring process. However, the reverse sequence of
procedure is also possible.
[0038] According to the invention, it is preferable to also provide
a step of lapping to polish off 10 .mu.m to 100 .mu.m, for example
before or after the step of coring of the recycled wafer. The step
of lapping after the step of coring may be provided, for example,
between the step of coring and the step of chamfering, between the
step of chamfering and the step of circumferential face-polishing,
or after the step of circumferential face-polishing. The step of
lapping may be provided preferably between the step of chamfering
and the step of circumferential face-polishing, or after the step
of circumferential face-polishing.
[0039] It is mostly possible to remove pits and defects from the
surface of the wafer base plate in the lapping step. It has been
found that if the pits and defects are able to be removed, there is
no effect on substrate strength. Defects and pits originating from
the various semiconductor processes undergone by the recycled wafer
are confined to a portion extremely close to the surface, so these
can be mostly removed by lapping away 10 .mu.m to 100 .mu.m off the
surface. The thickness of a 200 mm monitor wafer is 0.835 mm, and
the thickness of a discarded recycled wafer is 0.6 to 0.7 mm.
Because the standard thickness of a 65 mm substrate is 0.635 mm, it
is desirable that the HDD substrate of the invention is applied to
small diameter substrates of not more than 65 mm.
[0040] Furthermore, according to the invention, there is a reason
why the layer to be polished off for the removal of defects can be
comparatively thin. Although a HDD substrate uses both surfaces, a
semiconductor wafer basically uses only a single surface.
Accordingly, a rear surface gets by without undergoing various
processes (other than heating). Consequently, damage to the rear
surface is comparatively light so that it is possible to
concentrate on removal of defects from the used surface of the
recycled wafer. The majority of defects which cause a loss of
strength are removable by lapping 10 to 100 .mu.m. However, it
could be a problem if crystal defects or dislocations (the defect
frequency is greater than in the prime wafers or the like) existing
within the internal portion appear on surfaces. After polishing
both surfaces of the etched wafer following coring, defects on the
surfaces of the HDD substrate for causing a problem of decreased
substrate strength can be removed on basis of inspection of the
highly mirror finished surfaces. Hence, the problem is with defects
which exist on the circumferential faces.
[0041] In the fabrication of the HDD substrate shown in FIG. 1, a
step of chamfering of the inner and outer circumferential faces and
a step of the circumferential face-polishing may be provided after
the step of coring of the recycled wafer base plate.
[0042] The angle and dimensions for the chamfering may be for the
most part restricted to standard dimensions. When prime wafers or
monitor wafers are used as the base plate, the chamfering can
result in a finished product. However, when using the recycled
wafers of the invention as the base plates, internal defects and
the like which appear on the circumferential faces may work to
cause a reduction in substrate strength. The circumferential face
means the inner or outer circumferential lateral surface of the
doughnut-shaped substrate. Defects which appear on the
circumferential faces after the chamfering become a problem because
they may become starting points of substrate destruction. Providing
the step for removing distorted layers by etching after the step of
circumferential face-polishing following the step of chamfering,
the inventors have found that the substrate strength at a level
equivalent to that of monitor wafers can be ensured even when using
recycled wafer base plates.
[0043] After the step of circumferential face-polishing, or after
the step of lapping following the step of circumferential
face-polishing, it may be preferable that the substrate undergoes
further steps including a step of alkali etching, a step of
polishing the upper and lower surfaces of the substrate that has
been alkali-etched, and a subsequent step of washing.
[0044] The step of alkali etching for removing the deformations
caused by the steps of lapping and of circumferential
face-polishing, may be carried out by dipping in a 2 to 60 weight %
aqueous solution of sodium hydroxide at 40 to 60.degree. C. for
example.
[0045] The step of polishing the upper and lower faces of the
alkali-etched substrate can be carried out favorably in a method
known in the art. For example, a substrate mounted in a carrier
between an upper plate and a lower plate are clasped, rotated and
polished with colloidal silica as the polishing particles.
[0046] The step of washing can be carried out by brush washing
and/or a chemical washing using an alkali and/or an acid solution,
which is known in the art,
[0047] The substrate for a magnetic recording medium of the
invention can be treated in the same way as a conventional
substrate. For example, a soft magnetic layer and a recording layer
can be disposed on the substrate so as to be used as a
perpendicular magnetic recording medium. To increase adhesion of
the soft magnetic layer a primer layer can be formed prior to
forming the soft magnetic layer.
[0048] A protective layer and a lubricating layer can be formed
above the recording layer.
[0049] The invention will be explained based on examples below,
however the invention is not limited to them.
[0050] An overview of examples is given below.
[0051] A silicon monocrystalline wafer has surface orientation of
(100) and a diameter of 200 mm. The silicon monocrystalline wafer
which has been heated and/or etched in a semiconductor IC process
or the like, is lapped for removing 10 .mu.m to 100 .mu.m using
abrasive particles so that pits and defects are removed. Next,
doughnut-shaped circular substrates having an outer diameter of not
more than 65 mm are cut out from the wafer by laser light from a
laser light generating device, forming a plurality of substrates.
Next, the edges of the inner and outer circumferential faces of the
substrates are removed by a grindstone. The upper and lower faces
of the substrate are polished after alkali etching so that the
desired substrates are obtained. Finally, polishing material that
has adhered to the substrate is removed in the step of washing so
that production of the substrate is completed.
EXAMPLE 1
[0052] A wafer having a thickness of 0.61 mm and an outside
diameter of 200 mm which had been subjected to heating up to
1000.degree. C. four times was prepared. After removal of 50 .mu.m
by lapping, a doughnut-shaped circular substrate having a diameter
of 48 mm and an inner diameter of 12 mm was obtained by a YAG laser
processing device. Next, the edges of the inner and outer
circumferential faces of the substrate were removed by a grindstone
(diamond) and the circumferential faces were polished. After alkali
etching in a 50 wt % NaOH solution at 50.degree. C. for 20 minutes,
both surfaces of the substrate were polished with 5 wt % colloidal
silica until the mirror surface appears. Next, the polishing
material and the like which adhered to the substrate was removed in
the step of washing so that the magnetic recording medium substrate
was obtained. Compressive destructive strength was measured by
mounting the substrate for magnetic recording medium on a circle
end of a 45 mm diameter pipe, placing a 30 mm diameter Zr ball on
the substrate above the center of the pipe and adding a load from
top of the ball to the substrate using a load cell. It was 500 N
for average of five samples.
EXAMPLE 2
[0053] Apart from a lapping for removing 100 .mu.m, processing was
the same as in Example 1. When compressive destructive strength was
measured, it was 550 N for average of five samples.
EXAMPLE 3
[0054] A wafer having a thickness of 0.55 mm and an outside
diameter of 200 mm which had been heated up to 1000.degree. C. six
times and etched four times was prepared. After removal of 100
.mu.m by lapping, a doughnut-shaped circular substrate having a
diameter of 26 mm and an inner diameter of 7 mm was obtained by a
YAG laser processing device. Next, the edges of the inner and outer
circumferential faces of the substrate were removed with a
grindstone (diamond). After inner and outer circumferential
face-polishing and alkali-etching in a 50 wt % NaOH solution at
50.degree. C. for 20 minutes, both surfaces of the substrate were
polished with 5 wt % colloidal silica until the mirror surface
appears. Next, the polishing material and the like which adhered to
the substrate was removed in the step of washing so that the
substrate for a magnetic recording medium was obtained. The
compressive destructive strength was measured by mounting the
substrate for a magnetic recording medium obtained on a 20 mm
diameter pipe, and arranging a 10 mm diameter Zr ball on its inner
circumferential side. It was 70 N for average of five samples.
COMPARATIVE EXAMPLE 1
[0055] A wafer having a thickness of 0.74 mm and which had not
undergone heating or etching was prepared. After lapping for
adjusting the thickness and surface with abrasive particles, a
doughnut-shaped circular substrate having a diameter of 48 mm and
an inner diameter of 12 mm was obtained by a YAG laser processing
device. Next, the edges of the inner and outer circumferential
faces of the substrate were removed with a grindstone (diamond).
After inner and outer circumferential face-polishing and alkali
etching in a 50 wt % NaOH solution at 50.degree. C. for 20 minutes,
both surfaces of the substrate were polished with 5 wt % colloidal
silica until the mirror surface appears. Next, the polishing
material and the like which adhered to the substrate were removed
in the step of washing so that the substrate for a magnetic
recording medium was obtained. The compressive destructive strength
was measured by mounting the substrate for the magnetic recording
medium on a 45 mm diameter pipe, and arranging a 30 mm diameter Zr
ball on its inner circumferential side. It was 300 N for average of
five samples.
COMPARATIVE EXAMPLE 2
[0056] Apart from processing to a doughnut-shaped substrate having
a diameter of 26 mm and an inner diameter of 7 mm, processing was
the same as Comparative Example 1. The compressive destructive
strength was measured by mounting the substrate for a magnetic
recording medium on a 20 mm diameter pipe, and arranging a 10 mm
diameter Zr ball on its inner circumferential side. It was 50 N for
average of five samples.
[0057] As given above, it has been found that high strength can be
obtained at favorable efficiency when the substrate for a magnetic
recording medium is fabricated using a wafer which has undergone
heating and/or etching.
* * * * *